The present invention relates generally to gas turbine engines, and, more specifically, to exhaust nozzles therefor.
A typical military fighter aircraft includes a turbofan gas turbine engine having an augmenter or afterburner for producing additional thrust when desired. The engine has high specific thrust and operates at a high pressure ratio during takeoff for providing exceptional acceleration.
Since the engine operates over varying output thrust, the exhaust nozzle therefor is typically variable in the form of a converging-diverging (CD) exhaust nozzle. The nozzle throat varies in flow area between maximum and minimum values for maximizing performance of the engine throughout its entire flight envelope. The high pressure ratio combustion gases exhausted from the engine may be choked at the throat at sonic velocity, and are subsonic in the converging nozzle and supersonic in the diverging nozzle.
The typical CD exhaust nozzle merely directs the exhaust in the axial downstream direction for propelling the aircraft forward in flight. Improvements in aircraft performance may be obtained by turning the exhaust in different directions. For example, a Short Takeoff and Landing (STOL) and Short Takeoff/Vertical Landing (STOVL) aircraft may use an auxiliary or secondary exhaust nozzle for redirecting some or all of the exhaust flow obliquely to the engine axial centerline axis and up to perpendicular or normal thereto.
The secondary exhaust nozzle must be integrated with the engine outlet duct in a compact and aerodynamically efficient package. The secondary exhaust duct should be as short as possible while maximizing turning efficiency of the exhaust as it is discharged axially from the engine and diverted radially outwardly.
In one example, the secondary nozzle includes two dimensional (2-D) cascade turning vanes located at the discharge end of the exhaust duct. These cascade vanes may have a simple, symmetrical teardrop profile between which the exhaust is turned and redirected from the aircraft. These cascade vanes may also have a simple crescent airfoil profile in the direction of flow turning.
These types of cascade vanes are also found in thrust reversers for temporarily turning engine exhaust for use in braking aircraft speed during landings. Efficiency of cascade-vane thrust reversers in a typical commercial aircraft is not a critical design objective since they merely provide auxiliary braking, and since the engines operate at low pressure ratios.
However, for a military STOL or STOVL aircraft, cascade vane design is critical to operation since the engines have high specific thrust and high pressure ratios during takeoff, and are sized by the required performance of the secondary exhaust nozzle. An inefficient secondary exhaust nozzle requires a larger engine which is heavier and more expensive, and reduces the overall performance of the aircraft. An efficient secondary exhaust nozzle permits corresponding reduction in size of the engine and improves the overall performance of the aircraft.
Accordingly, it is desired to provide an improved secondary exhaust nozzle having more efficient cascade vanes for improving performance of the aircraft.